diff --git a/src/lib/ebus/symbol.h b/src/lib/ebus/symbol.h
index 997df7024..e3d0f489c 100755
--- a/src/lib/ebus/symbol.h
+++ b/src/lib/ebus/symbol.h
@@ -330,7 +330,7 @@ class SymbolString {
    * Return whether the byte sequence is complete with regard to the header and length field.
    * @return true if the sequence is complete.
    */
-  bool isComplete() {
+  bool isComplete() const {
     size_t lengthOffset = (m_isMaster ? 4 : 0);
     if (m_data.size() < lengthOffset + 1) {
       return false;